1. Field of the Invention
The present invention relates to a donor substrate for a laser induced thermal imaging (LITI) method and a method of fabricating an organic light emitting display device using the same, and more particularly, to a donor substrate for an LITI method, which may be easily fabricated and in which an organic layer has improved pattern characteristics, and a method of fabricating an organic light emitting display device using the same.
2. Description of the Related Art
Nowadays, with the arrival of a high-level information society, a consumer's demand for obtaining information rapidly and correctly in hand is increasing. In order to meet this demand, the development of various display devices, which are thin and light to be easily carried and operable in a high information processing rate, has been accelerated. As one example of such display devices, an organic light emitting display device (OLED) is attracting attention as a next generation display. Since, the OLED is an emissive display device in which, when a voltage is applied to an organic layer including an organic emission layer, electrons and holes are recombined in the organic emission layer to emit light, the OLED does not need a backlight unit unlike a liquid crystal display device (LCD), so its thickness and weight may be easily reduced and its fabrication process may be simplified. In addition, the OLED has other advantages such as a fast response speed nearly equal to that of a cathode ray tube (CRT), low voltage driving, high luminous efficiency, and a wide viewing angle.
The OLED may be a small molecular OLED or a polymer OLED according to the material of the organic layer, in particular, the organic emission layer.
The small molecular OLED includes multiple organic layers having different functions from each other, which are interposed between an anode and a cathode, wherein the multiple organic layers include a hole injection layer, a hole transport layer, an emission layer, a hole blocking layer and an electron injection layer. These layers may be adjusted by doping to prevent the accumulation of electric charges or replacing with a material having a suitable energy level. The small molecular OLED is generally made by a vacuum deposition method and thus it is difficult to realize a large-sized display.
On the other hand, the polymer OLED has a single layer structure having an organic emission layer interposed between an anode and a cathode or a double layer structure including a hole transport layer in addition to the organic emission layer, and thus may be fabricated into a thin device. In addition, since the organic layer is formed by a wet coating method, the polymer OLED may be fabricated under atmospheric pressure, thereby reducing the manufacturing cost and readily realizing the large-sized OLED.
In the case of a monochrome device, the polymer OLED may be simply fabricated by a spin coating method, but has disadvantages of lower efficiency and shorter lifetime compared to the small molecular OLED. In the case of a full color device, emission layers for showing three primary colors of red, green and blue may be patterned in such an OLED to realize the full color. In this case, the organic layer of the low small OLED may be patterned by a shadow mask deposition method, and the organic layer of the polymer OLED may be patterned by an ink jet printing method or a laser induced thermal imaging (hereinafter will be referred to as “LITI”) method. The LITI method may utilize spin coating characteristics as they are, thereby resulting in excellent internal uniformity of pixels in the large-sized OLED. In addition, since the LITI method adopts a dry process instead of a wet process, the LITI method may prevent lifetime reduction by solvent as well as realize a fine pattern in the organic layer.
Application of the LITI method basically needs a light source, an OLED substrate (hereinafter will be referred to as “substrate”) and a donor substrate, wherein the donor substrate includes a base layer, a light-to-heat conversion layer and a transfer layer. According to the LITI method, light emitted from the light source is absorbed by the light-to-heat conversion layer to convert the light into heat energy, so that an organic material formed on the transfer layer is transferred onto the substrate by the converted heat energy.
FIG. 1 is a cross-sectional view for explaining a general transfer mechanism for transfer patterning an organic layer used in an organic light emitting display device (OLED) using a laser.
As shown in FIG. 1, in the transfer mechanism for transfer patterning the organic layer used in the organic light emitting display device (OLED) using the laser, an organic layer S2 attached to a substrate S1, which includes a base layer 11 and a light-to-heat conversion layer 12, is detached from the substrate S1 in response to a laser beam and transferred to a substrate S3, which includes a substrate 22 and a pixel defining layer 21, to be separated from a portion in which the laser beam is not irradiated.
Transfer characteristics are determined by a first adhesion W12 between the substrate S1 and the organic layer S2, a cohesion W22 between the organic layer S2 and a second adhesion W23 between the organic layer S2 and the substrate S3.
The first and second adhesions and the cohesion are summarized as the following Equation:W12=γ1+γ2−γ12W22=2γ2W23=γ2+γ3−γ23                where γ1, γ2 and γ3 are surface tensions of S1, S2 and S3, respectively, γ12 is the interfacial tension between S1 and S2, and γ23 is the interfacial tension between S2 and S3.        
In order to improve the characteristics of the LITI, the cohesion of the organic layer should be smaller than the adhesions between the organic layer and the substrates.
In the case that the organic layer is made of a small molecular material, the first and second adhesions are larger than the cohesion of the organic layer so that the small molecular material is easily transferred from a donor substrate 14 onto a substrate 20. However, by virtue of the small first adhesion, other portions of the small molecular material layer, which are not exposed to the laser beam, may be disadvantageously transferred onto the substrate 20. Alternatively, in the case that the organic layer is made of a polymer material, uniform patterning may be difficult because of high cohesion of the polymer material.